Given Zirconium alloy prevalent application as a nuclear material, it is vital to illustrate its deformation mechanism. In this study, in-situ tensile tests of the CZ1 (Zr–Sn–Nb) alloy were conducted at various temperatures (25 °C and 430 °C) using electron backscatter diffraction (EBSD). During in-situ tensile process, slip traces, grain deformation, as well as texture evolution were characterized at the deformation of 2%, 6% and 10%, respectively. In randomly selected grains with 10% strain, the Schmid factor and slip traces were used to determine the activation of slip system. At all temperatures, the prismatic < a > slip is the main slip system in most grains. Comparing the tensile samples at room temperature and high temperatures, the results shown that the pyramidal slip plays a role in coordinating the plastic deformation, and the role of pyramidal slip increases with the increase of temperature.